A variety of containers are used to package substances which are sensitive to air to ensure extended shelf-life of the contained product. The seal for the lid of the container must be complete to ensure viability of the contents. This is particularly important in the field of packaging food and other comestibles and medical products and the like. Containers, which are particularly sensitive to defects in container seal, are those containers where a lid is welded or glued to the container rim. Such types of containers are used for packaging a variety of food products, such as meal portions, pet foods, dairy products and the like, as well as liquids usually in the form of medications and cosmetics.
There are presently available a variety of techniques which may be used to test the integrity of the container seal to ensure extended shelf life of the contained product. Such testing procedures include application of a pressure differential to the container exterior and thereby ascertain whether or not there are leaks at the bond of the lid to the container. The pressure differential may be applied by either drawing a vacuum on or applying pressure to the container. This testing procedure only works with rigid or semi-rigid containers. Soft containers are either crushed or blowout. Container defects may also be determined by way of costly visual inspection which cannot check internal aspects of the seal. X-ray imaging may also be used, but is of questionable worth and does not work with aluminum laminates.
It has been suggested in the publication R and D, Selman, J. D. "On-Line Detection of Food Container Faults" February, 1987, that optical equipment could be used to provide measurement of the seam dimensions by use of fibre optics systems or lasers to detect seal faults. Thermo-imaging is another technique that may have application in determining flaws in the seal, but it has been found to be too insensitive. The thermal properties of trapped defects, such as meat fibres and bond line materials, are similar. It is also suggested in this reference that ultrasonics could be used to measure the internal seam bond structure of the lidded container. There is no discussion, however, in the reference of the manner in which ultrasonic testing equipment could be used in detecting flaws in the seal of the container.
Published U.K. patent application 2,029,960 (Mar. 26, 1980) discloses an ultrasonic testing device for testing claddings, solder joints, adhesive points other types of surface connections. The device is particularly suited for testing solder joints. A transmitting transducer is coupled to the bonded surface via coupling medium. On the other side of the bond, a receiving transducer is coupled via a coupling medium. Adaptors may be used to conform to the geometric shape of the bonded item. The received ultrasonic sound is converted into a signal which is displayed by the apparatus as a sound intensity. A drop in sound intensity indicates that the bond is not complete and hence faulty.
Langrock et al disclose in a report from the Central institute for Welding Technology of the DDR in Halle entitled "Possibilities of Testing Lap-Welded Joints in Thermoplastic Sealing Strips" that a reflective form of electrosonic tester unit would be useful in detecting faults in lap-joints of plastic material. Pulse wave forms generated by the testing equipment are analyzed to detect defects in the welded lap-joint of similar plastic materials. The pulse echo mode of operation is effective with this type of lap-joint in view of the thickness of the films which can be in excess of 1 to 2 mm. By analyzing the reflection of the ultrasonic energy, evaluation can be made in the uniformity of the lap-joint in the sealed plastic surfaces. However, the echo system would not be sufficiently sensitive to detect very fine defects in the joint, such as experienced in containers where meat fibers and other food fibers can be caught between the bond of the lid to the container which constitutes forming a defective seal. Compared to these thicknesses, the containers in question have a welded plastic thickness of only 15 .mu. with a total seam thickness of only 185 .mu..